Perhaps U.S. Attorney Chris Christie, a Republican, is trying to sabotage the Democratic primary for U.S. Senate. Then this would all make sense.

But otherwise I can't figure out just what Christie was up to Sunday, when he publicly picked a fight with Democratic Morristown Mayor Don Cresitello over the question of illegal immigration.

Christie used the occasion of a forum at a church in Dover to renew a very public fight he and Cresitello had arising out of a rally the mayor held last July at the town hall. Hundreds of people gathered on the lawn to hear Cresitello and the other speakers call for local authorities to gain powers under a 1996 federal statute that permits local law enforcement to assist the feds in enforcing immigration law.

Meanwhile, hundreds of protesters gathered across the street. When the rally organizers played the national anthem, the protesters tried to drown it out with various jeers and catcalls. This led Cresitello to denounce the "Marxists and communists" across the street.

There were indeed Marxists over there in front of Kings supermarket. I saw the various red banners with my own eyes. So did Mike Carroll, who is a state assemblyman from Morris County and a Republican.

"Cresitello's comments were not directed at the immigrants," recalls Carroll. "They were directed at the Socialist Workers Party."

In the days after the rally, however, Christie denounced Cresitello for "grandstanding" by denouncing the Marxists. The dispute simmered over the next few weeks and then flared again in August, when four college kids in Newark were shot by a group of thugs whose leader could have been deported under the very law Cresitello was seeking to enforce.

Cresitello looked like a prophet. And Christie looked to be at a loss.

So it was a bit unusual the other day when Christie chose to rehash the whole affair at a church in Dover. Christie didn't single out Cresitello by name, but he did allude to "certain leaders around the state that have demagogued on this issue."

In case anyone was wondering just who those certain leaders are, Christie went on to state that "I don't think it's helpful when a mayor of a town in New Jersey stands up at a rally and calls people he believes to be undocumented 'pinkos' and communists."

For good measure, Christie also said that "being in this country without proper documentation is not a crime."

This is technically true. But Christie caught a lot of heat for seeming to be soft on illegal immigration.

Meanwhile the mayor got a much-needed burst of attention in his lagging campaign for U.S. Senate. Until this week, Cresitello was not even an also-ran in the race to unseat Frank Lautenberg. News reports have treated it as a two-way race between the 84-year-old Lautenberg and 50-year-old Rob Andrews, a congressman from South Jersey.

When I phoned the mayor, he was basking in the glow of that publicity, which included an interview with TV's biggest border defender, Lou Dobbs.

"There's a ton of Democrats, both liberal and conservative, who support me on this issue on both homeland security grounds and because you're violating the law if you work here illegally," said Cresitello.

His campaign manager, lawyer Paul Bangiola, told me that the publicity opens all sorts of fundraising possibilities for a campaign that can sure use the dough. And now Cresitello has got a good argument for demanding a place at the table in any Democratic debates, if Lautenberg ever agrees to them.

The big question at that point would be whether Cresitello takes votes out of Lautenberg's geographical base in North Jersey or out of the anti-incumbent vote that Andrews hopes to capture. So perhaps this is all a brilliant and fiendish plot by the Republican U.S. attorney to throw a monkey wrench into the Democratic primary.

Or perhaps not. More likely, Christie was just trying to shore up his left flank in what is expected to be his run for governor next year.

If that's the case, this could be a major mistake. At the moment, Christie's most likely primary opponent is former Bogota mayor Steve Lonegan. Lonegan, who ran in the 2005 gubernatorial primary, is riding high after being one of the most outspoken opponents of Gov. Jon Corzine's now-dead toll-hike scheme. And he's well to the right of even Cresitello on illegal immigration.

"I don't think anyone else is going to say that coming into the country illegally is not a crime," said Lonegan. "I mean, it's pretty black and white."

Well, it's black and white to most Republicans. Looks like a gray area for Christie, though.